ASTM E426 - 16 (2021)

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Designation: E426 −16 (Reapproved 2021)

Standard Practice for

Electromagnetic (Eddy Current) Examination of Seamless

and Welded Tubular Products, Titanium, Austenitic Stainless

Steel and Similar Alloys

This standard is issued under the ﬁxed designation E426; the number immediately following the designation indicates the year of

original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A

superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

This standard has been approved for use by agencies of the U.S. Department of Defense.

1. Scope*

1.1 This practice

covers procedures for eddy current ex-

amination of seamless and welded tubular products made of

relatively low conductivity materials such as titanium, stainless

steel, and similar alloys, such as nickel alloys. Austenitic

chromium-nickel stainless steels, which are generally consid-

ered to be nonmagnetic, are speciﬁcally covered as distin-

guished from the martensitic and ferritic straight chromium

stainless steels which are magnetic.

1.2 This practice is intended as a guide for eddy current

examination of both seamless and welded tubular products

using either an encircling coil or a probe-coil technique. Coils

and probes are available that can be used inside the tubular

product; however, their use is not speciﬁcally covered in this

document. This type of examination is usually employed only

to examine tubing which has been installed such as in a heat

exchanger.

1.3 This practice covers the examination of tubular products

ranging in diameter from 0.125 to 5 in. (3.2 to 127.0 mm) and

wall thicknesses from 0.005 to 0.250 in. (0.127 to 6.4 mm).

1.4 For examination of aluminum alloy tubular products,

see standard Practice E215.

1.5 Units—The values stated in inch-pound units are to be

regarded as standard. The values given in parentheses are

mathematical conversions to SI units that are provided for

information only and are not considered standard.

1.6 This standard does not purport to address all of the

safety concerns, if any, associated with its use. It is the

responsibility of the user of this standard to establish appro-

priate safety, health, and environmental practices and deter-

mine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accor-

dance with internationally recognized principles on standard-

ization established in the Decision on Principles for the

Development of International Standards, Guides and Recom-

mendations issued by the World Trade Organization Technical

Barriers to Trade (TBT) Committee.

2. Referenced Documents

2.1 ASTM Standards:

E215 Practice for Standardizing Equipment and Electromag-

netic Examination of Seamless Aluminum-Alloy Tube

E543 Speciﬁcation for Agencies Performing Nondestructive

Testing

E1316 Terminology for Nondestructive Examinations

2.2 Other Documents:

SNT-TC-1A Recommended Practice for Personnel Qualiﬁ-

cation and Certiﬁcation in Nondestructive Testing

ANSI/ASNT CP-189 ASNT Standard for Qualiﬁcation and

Certiﬁcation of Nondestructive Testing Personnel

NAS-410 NAS Certiﬁcation and Qualiﬁcation of Nonde-

structive Personnel (Quality Assurance Committee)

ISO 9712 Non-Destructive Testing—Certiﬁcation and

Qualiﬁcation of NDT Personnel

3. Terminology

3.1 Standard terminology relating to electromagnetic testing

may be found in Terminology E1316, Section C, Electromag-

netic Testing.

This practice is under the jurisdiction of ASTM Committee E07 on Nonde-

structive Testing and is the direct responsibility of Subcommittee E07.07 on

Electromagnetic Method.

Current edition approved June 1, 2021. Published June 2021. Originally

approved in 1971. Last previous edition approved in 2016 as E426 – 16. DOI:

10.1520/E0426-16R21.

For ASME Boiler and Pressure Vessel Code applications see related Practice

SE-426 in Section II of that Code.

For referenced ASTM standards, visit the ASTM website, www.astm.org, or

contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM

Standards volume information, refer to the standard’s Document Summary page on

the ASTM website.

Available from American Society for Nondestructive Testing (ASNT), P.O. Box

28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.

Available from Aerospace Industries Association of America, Inc. (AIA), 1000

Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org.

Available from International Organization for Standardization (ISO), ISO

Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,

Geneva, Switzerland, http://www.iso.org.

*A Summary of Changes section appears at the end of this standard

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the

Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

4. Summary of Practice

4.1 The examination is conducted using one of two general

techniques shown in Fig. 1. One of these techniques employs

one or more exciter and sensor coils which encircle the pipe or

tube and through which the tubular product to be examined is

passed. Some circuit conﬁgurations employ separate exciter

and sensor coils; whereas other conﬁgurations employ one or

more coils that concurrently function as both exciters and

sensors. Alternating current passes through the exciting coil

which by reason of its proximity induces current in the tubular

product. The sensor coil detects the resultant electromagnetic

ﬂux related to these currents. The presence of discontinuities in

the tubular product will affect the normal ﬂow of currents and

this change is detected by the sensor. The encircling coil

technique is capable of examining the entire 360-deg expanse

of the tubular product.

4.2 Another technique employs a probe coil with one or

more exciters and sensors which is brought in close proximity

of the surface of the tubular product to be examined. Since the

probe is generally small and does not encircle the article being

examined, it examines only a limited area in the vicinity of the

probe. If it is desired to examine the entire volume of the

tubular product, it is common practice to either rotate the

tubular product or the probe. In the case of welded tubular

products frequently only the weld is examined by scanning

along the weld zone. In the case where the tubular products are

joined by welding and the probe is rotated, the probe is orbited

about the central axis of the tube such that a circumferential

examination of the tube and/or weld may be made. The depth

of penetration of the interrogating magnetic ﬁelds into the

tubular product may be smaller for this type of probe coil

compared to the encircling coil.

5. Signiﬁcance and Use

5.1 Eddy current testing is a nondestructive method of

locating discontinuities in a product. Changes in electromag-

netic response caused by the presence of discontinuities are

detected by the sensor, ampliﬁed and modiﬁed in order to

actuate audio or visual indicating devices, or both, or a

mechanical marker. Signals can be caused by outer surface,

inner surface, or subsurface discontinuities. The eddy current

examination is sensitive to many factors that occur as a result

of processing (such as variations in conductivity, chemical

composition, permeability, and geometry) as well as other

factors not related to the tubing. Thus, all received indications

are not necessarily indicative of defective tubing.

6. Basis of Application

6.1 If speciﬁed in the contractual agreement, personnel

performing examinations to this practice shall be qualiﬁed in

accordance with a nationally recognized NDT personnel quali-

ﬁcation practice or standard such as ANSI/ASNT-CP-189,

SNT-TC-1A, NAS-410, ASNT-ACCP, ISO 9712, or a similar

document and certiﬁed by the certifying agency, as applicable.

The practice or standard used and its applicable revision shall

be identiﬁed in the contractual agreement between the using

parties.

NOTE 1—MIL-STD-410 is canceled and has been replaced with

NAS-410, however, it may be used with agreement between contracting

parties.

6.2 If speciﬁed in the contractual agreement, NDT agencies

shall be qualiﬁed and evaluated in accordance with Speciﬁca-

tion E543. The applicable edition of Speciﬁcation E543 shall

be speciﬁed in the contractual agreement.

7. Apparatus

7.1 Electronic Apparatus—The electronic apparatus shall be

capable of energizing the examination coils or probes with

alternating currents of suitable frequencies and shall be capable

of sensing the changes in the electromagnetic response of the

sensors. Equipment may include a detector, phase

discriminator, ﬁlter circuits, modulation circuits, magnetic-

saturation devices, recorders, and signaling devices as required

for the particular application.

7.2 Examination Coils—Examination coils shall be capable

of inducing current in the tube and sensing changes in the

electrical characteristics of the tube.

NOTE 2—Fill factor effect is an important consideration since coupling

variations can affect the examination signiﬁcantly.

7.3 Probe Coils—Probe coils shall be capable of inducing

current in the tube and sensing changes in the electrical

characteristics of the tube (Note 3). Probes generally consist of

FIG. 1 Sketch Showing Encircling-Coil and Probe-Coil Tech-

niques for Electromagnetic Examination of Tubular Products

E426 − 16 (2021)

an exciting coil and sensing coil or Hall element mounted in a

common holder. A Hall element is a semiconductor that by

reason of the Hall effect is capable of responding in a manner

directly proportional to magnetic-ﬂux density. However, when

used with an exciting coil, it should be remembered that eddy

current ﬂow is inﬂuenced by the excitation frequency.

NOTE 3—Lift-off effect is an important consideration since coupling

variations can affect the examination signiﬁcantly.

7.4 Driving Mechanism—A mechanical device capable of

passing the tube through the examination coil or past the probe.

It shall operate at a uniform speed with minimum vibration of

coil, probe, or tube and maintain the article being examined in

proper register or concentricity with the probe or examination

coil. Where required, the mechanism shall be capable of

uniformly rotating the tube or probe.

7.5 Reference Standard—The standard used to adjust the

sensitivity setting of the apparatus shall be sound and of the

same nominal alloy, temper, and nominal dimensions as the lot

of tubes or pipes to be examined on a production basis. It shall

be of sufficient length to permit the required spacing of the

artiﬁcial discontinuities (at least 4 ft, and preferably longer).

Artiﬁcial discontinuities made in the tube or pipe shall be

centered as nearly as possible on one inside or outside diameter

surface of the tube and shall preferably be of one of the

following types:

7.5.1 Holes—Holes that are usually drilled completely

through the wall may be used. Care should be taken during

drilling to avoid distortion of the tube (or pipe) and hole.

7.5.2 Notches—Notches may be produced by electric dis-

charge machining (EDM), milling, or other means. Longitudi-

nal or transverse notches or both may be used (Note 4).

Orientation, dimensions (width, length, and depth), and con-

ﬁguration of the notches affect the response of the eddy current

system. Notch depth is usually speciﬁed as a percentage of

nominal wall thickness of the tubular product being examined.

Notches may be placed on the outer, inner, or both surfaces of

the reference (calibration) standard. Outer surface notches

provide an indication of system response to discontinuities

originating on the outer tube surface, whereas inner surface

notches provide an indication of system response to disconti-

nuities originating on the inner tube surface (Note 5).

NOTE 4—Longitudinal notch standards are normally used when exam-

ining with rotating probe systems.

NOTE 5—The density of eddy currents decreases nearly exponentially

with increasing distance from the surface nearest the coil, and the

sensitivity to subsurface discontinuities decreases with the change in depth

allowing the use of phase analysis techniques.

7.5.3 The conﬁguration, orientation, and dimensions (diam-

eter of holes and the width, length, and depth of notches) of the

artiﬁcial discontinuities to be used for establishing acceptance

limits should be subject to agreement between supplier and

purchaser.

8. Adjustment and Standardization of Apparatus

Sensitivity

8.1 Select the apparatus, examination frequency, coil or

probe, or both, design, phase discrimination, and other circuitry

as well as speed of examining which shall demonstrate the

system capability for detecting the discontinuities of interest.

8.2 Fabricate the applicable reference standard in accor-

dance with the agreement between the purchaser and tubing

supplier.

8.3 Adjust the apparatus to obtain an optimum signal-to-

noise ratio with the minimum sensitivity required to detect the

artiﬁcial discontinuities in the reference standard. Do this

under conditions (such as examining speed) identical to those

to be used in production examination of the tubular products.

8.4 Determine the end effect by using a special reference

tube or pipe containing a series of notches or holes near one or

both of the ends and passing this reference standard through the

system at production examination speeds. If notches or holes

are placed near only one of the ends, pass the tube through the

system backwards and forwards.

9. Procedure

9.1 Standardize the apparatus at the start of the examination

run using the reference standard. The recommended maximum

interval between re-standardization is four hours although

more or less frequent re-standardization may be done by

agreement between using parties, or whenever improper func-

tioning of the equipment is suspected. If improper functioning

is found, re-standardize the apparatus and re-examine all tubes

or pipes examined during the period since the last successful

standardization.

9.2 Pass the lot of tubes or pipes to be examined through the

examination coil or past the probe coil of the apparatus

adjusted to the sensitivity as described in Section 8. Set aside

tubes or pipes with discontinuities indicated by the apparatus.

It is recommended that tubes with discontinuity indications be

re-examined in accordance with the purchase speciﬁcation.

9.3 Tubes or pipes may be examined in the ﬁnal drawn,

annealed, heat treated, as-welded, or other step in processing.

The point in processing at which examination is made should

be agreed upon by the supplier and the purchaser. The tubes

should be free of any substance that may interfere with the

examination.

10. Supplemental Information Regarding Eddy Current

Examination of High Alloy Steels and Similar Alloys

10.1 In the eddy current examination of austenitic

chromium-nickel stainless steels it has been found that exami-

nation frequencies ranging from less than 1 kHz to more than

1 MHz can be used. The more commonly used operating

frequencies are in the range, 1 kHz to 125 kHz. The exact

frequency used will depend on the application.

Example—If thin-walled tubular products are to be exam-

ined or surface rather than subsurface discontinuities are to be

detected, higher frequencies are used. For heavy-walled tubes,

subsurface discontinuities well below the surface require the

use of a lower frequency. Choice of examination frequency

will determine the size of discontinuity that can be detected.

10.2 As in any eddy current examination, the depth and

orientation of the discontinuity below the entry surface will

E426 − 16 (2021)

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摘要：

ASTM E426 - 16 (2021) 是国际材料与试验协会发布的一项标准规范，专门用于通过电磁（涡流）测试方法对无缝和焊接的黑色金属管材进行检测，旨在识别管材表面及近表面的缺陷，如裂纹、折叠、夹杂物等。该标准适用于直径从6毫米到超过200毫米的钢管，涵盖了测试设备、校准程序、测试参数设置以及结果评估的详细要求，确保检测结果的一致性和可靠性。2021年的修订版本更新了技术参考文献和操作指导，使其更符合现代工业检测实践。对于从事钢管制造、质量控制和认证的专业人士而言，遵循ASTM E426 -

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